Biotin carboxyl carrier protein domain

Fig. 1. Crystal structure of the biotin carboxyl carrier protein domain of Escherichia coli. The N- and C-termini and, 50A away, the single lysine residue that is biotinylated in vivo can all clearly be seen. Figure prepared from PDB file 1BDO using Swiss PDB Viewer (19).

Fig. 2. Acetyl-CoA carboxylase. (A) Eukaryotic ACCs contain -2300 residues organized into three functional domains — biotin carboxylase (BC), biotin carboxyl carrier protein (BCCP), and carboxyltransferase (CT). The role of the region between the biotin carboxyl carrier and carboxyltransferase domains is unknown. The biotin carboxyl carrier protein contains a typical conserved biotin attachment-site motif, VMKMV. The sites of phosphorylation are indicated by asterisks. (B) Electron micrograph of polymerized rat acetyl-CoA carboxylase (F. Ahmad, 1978). (C) Crystal structure of the biotin carboxylase domain of the yeast enzyme. In the presence of soraphen A, the biotin carboxyl carrier protein domain forms an inactive monomer the likely position of the modeled ATP-binding site is shown (adapted from Ref. [2]). (D) Crystal structure of the dimeric carboxyltransferase domain of the yeast enzyme. Although acetyl-CoA was included in the crystallization, density was observed only for CoA at one site and adenine at the other (adapted from Ref. [2]). (E) NMR structure of the biotin carboxyl carrier apoprotein domain of the human ACC2 The lysine attachment site for biotin is shown (RIKEN Structural Genomics/Proteomics Initiative, 2006). (See color plate section, plate no. 3.)...

Each ACC half-reaction is catalyzed by a different protein sub-complex. The vitamin biotin is covalently coupled through an amide bond to a lysine residue on biotin carboxyl carrier protein (BCCP, a homodimer of 16.7-kDa monomers encoded by accB) by a specific enzyme, biotin-apoprotein ligase (encoded by birA), and is essential to activity. The crystal and solution structures of the biotinyl domain of BCCP have been determined, and reveal a unique thumb required for activity (J. Cronan, 2001). Carboxylation of biotin is catalyzed by biotin carboxylase (encoded by accC), a homodimeric enzyme composed of 55-kDa subunits that is copurified complexed with BCCP. The accB and accC genes form an operon. The three-dimensional structure of the biotin carboxylase subunit has been solved by X-ray diffraction revealing an ATP-grasp motif for nucleotide binding. The mechanism of biotin carboxylation involves the reaction of ATP and CO2 to form the shortlived carboxyphosphate, which then interacts with biotin on BCCP for CO2 transfer to the I -nitrogen. 

Fig. 1. Structure of the coli biotin carboxyl carrier protein (BCCP) domain. Residues 77-156 are drawn (coordinate file Ibdo), showing the N- and C-termini and the single biotin moiety that is attached to lysine 122 in vivo by biotin ligase. Representation produced using SwissPDBViewer (23).

Figure 1 shows the domain order of ACC from Arabidopsis. The domains order from the N-terminus is CARBOXYTRANSFERASE, BIOTIN CARBOXYL CARRIER PROTEIN and TRANSCARBOXYLASE. This is the same order as the domains in animal ACC. The data has been deposited in the EMBL DATA base (Fig. 1c). Recent reports indicate that in plants two forms of ACC exist and the cloned form represents a cytoplasmic form. Anti-sense constructs to the enzyme are currently being used in transgenic studies to identify the importance of this enzyme.

In prokaryotes and in plastids of some plants, the ACC is a multisubunit enzyme, whereas in eukaryotes the cytosolic isozyme and, in some instances also the plastid isozyme, are multidomain proteins. The latter contain three major functional domains, which account for the biotin carboxylase (BT), biotin carboxyl-carrier (BCC) and carboxyltransferase (CT) activities and, which are organized in one large polypeptide. 

Acetyl-CoA carboxylase (ACC) catalyzes the first committed step in long-chain fatty acid biosynthesis (see Chapter 7.11). The overall reaction is catalyzed in two sequential reactions (Scheme 3). First, the biotin carboxylase domain catalyzes the ATP-dependent carboxylation of biotin (which is attached to a carrier protein) using bicarbonate as a CO2 donor. In the second reaction, the carboxyl group is transferred from biotin to acetyl-CoA to form malonyl-CoA. In mammals, both reactions are catalyzed by a single protein, but in Escherichia coli and other bacteria, the activity is catalyzed by two separate proteins, a biotin carboxylase and a carboxytransferase. Due to its role in fatty acid synthesis, inhibitors of the overall ACC reaction are proposed to be useful as antiobesity drugs in mammals as well as novel antibiotics against bacteria. 

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